1. Field of the Invention
The present invention relates to copiers, and more particularly, to apparatus and method for improving the contrast and density of copier copies.
2. Description of the Prior Art
It is well known that the contrast of copies can be adjusted by changing copier operation. The term contrast, as used herein, refers to the rate of change (or slope) of the output copy density Dout with respect to the input copy density Din.
In one control technique (closed-loop), the charge on a photoconductor is measured and the contrast and density of copies adjusted by varying one or more of the copier parameters. See commonly assigned U.S. Pat. No. 3,779,204, issued Dec. 18, 1973 and U.S. Pat. No. 4,087,171, issued May 2, 1978. Although this technique is useful, it is sometimes difficult to accurately measure the charge or voltage on a photoconductor.
A copier "open-loop" control technique is more frequently used. In this technique, the copier can selectively be operated at one of three sets which correspond to normal, lighten, or darken copies. Normal, lighten, or darken copies generally refers to copy density relative to input document density. For example, when a lighten copy is designated, copies are made lighter than original documents. The open-loop technique depends on the stability of all the copier parts to maintain a consistently high-quality output. Since the response of the copier parts is not always identical, let alone perfectly stable, some compromise is made during setup to allow variations to occur without causing objectionable defects in copies. For example, as developer ages, it causes changes in copy density. Consequently, in the setup for normal copy operation, exposure is usually set intentionally light (overexposed) to prevent any possible appearance of background as the developer ages. In this situation, line copy and solid area density are often not optimal. Due to manufacturing tolerances, different batches of toner and developer may have different contrasts. The term toning contrast is defined later in this specification. Suffice it here to say, since most copiers operate with fixed parameters, copy density may vary with toner or developer batches having different toning contrasts and occasionally be objectionably low or high.
With a given concentration of a particular toner in a developer, there are three copier parameters that usually are varied to change copy contrast and density. They are the voltage applied onto the photoconductor by the primary charger, the copier exposure, and the bias voltage applied to the development station. The bias voltage has the same polarity as the electrostatic image and affects toner deposition. Varying the bias voltage provides some control of copy contrast and density (especially minimum copy density).
These three are interrelated and their proper adjustment by an operator would require considerable skill and judgment on his part. Consequently, copier manufacturers have elected to provide an operator with only limited control over copy contrast and density.